An internal combustion engine draws air into cylinders via an induction system and into an intake manifold through an induction pipe that is regulated by a throttle valve. For turbocharged applications, the induction system comprises a compressor of a turbocharger that forces air into through the induction pipe and into the intake manifold. The air in the intake manifold is distributed to a plurality of cylinders via respective intake valves and is combined with fuel to create an air/fuel mixture. The air/fuel mixture is combusted (e.g., by spark from respective spark plugs) within the cylinders to drive pistons that generate torque at a crankshaft. The exhaust gas resulting from combustion is expelled from the cylinders and into an exhaust system via respective exhaust valves. For turbocharged applications, the kinetic energy of the exhaust gas drives a turbine of the turbocharger (which in turn drives the compressor via a shaft) and the exhaust gas is treated by an exhaust treatment system to decrease emissions prior to its release into the atmosphere.
Scavenging refers to the operation of the engine such that opening of the intake and exhaust valves overlaps, and the cylinder delta pressure between intake and exhaust forces the air charge to blow through the cylinder and exit via the exhaust valve. Scavenging operation is capable of increasing engine performance. For turbocharged applications, this is true particularly at certain operating conditions such as low engine speeds where exhaust energy available for the turbocharger is low. The scavenging ratio represents a ratio of the total air charge flowing through each cylinder to an air charge trapped in each cylinder. For example, a scavenging ratio of 1.10 indicates that 10% of the total air charge blows through the cylinder or rather is not trapped. An inverse of the scavenging ratio represents a trapping efficiency. For example, a scavenging ratio of 1.10 corresponds to a trapping efficiency of ˜91%. The scavenging ratio/trapping efficiency of an engine is utilized for a variety of important controls. Non-limiting examples of engine controls that utilize the scavenging ratio include airflow, fueling, spark timing, and emissions controls.
Conventional techniques for estimating the scavenging ratio of the engine, however, are inaccurate, which results in inaccurate controls. One conventional technique involves comparing airflow to engine torque output. Not all of an engine's airflow, however, is converted to torque, e.g., due to varying combustion characteristics contributed by various air/fuel ratio, spark timing, and other factors. Another conventional technique involves measuring oxygen in the exhaust gas. The reason that using an oxygen sensor for scavenging ratio measurement is not ideal is because the oxygen sensor reading will be corrupted due to a mixture of air and fuel that is present at the sensor. That is, the sensor operates well when only air or fuel is present; however, when both air and fuel are present at the sensor (which is true while scavenging is active), the sensor reading (air/fuel ratio) no longer represents the true value, and thus cannot be trusted. Accordingly, while such scavenging ratio estimation systems work well for their intended purpose, there remains a need for improvement in the relevant art.